A flash memory is a typical example of a non-volatile semiconductor memory. Data can be freely written in and erased from a flash memory and the data content is not erased even when power supply is cut. This flash memory offers the advantages, of both RAM (Random Access Memory) and ROM (Read Only Memory). For this reason, recently, flash memory is used packaged in a card form as a memory card in a recording medium of a mobile phone, a digital camera, a portable music player and so forth.
In such a flash memory, a memory able to store two bits of information in a single memory cell has been considered in order to improve the level of integration of the flash memory. For example, in Japanese Translation of PCT International Application, Publication No. 2001-512290, an EEPROM (Electrical Erasable Programmable Read Only Memory), which is a programmable read-only memory, is disclosed.
FIG. 1 and FIG. 2 show a configuration of a memory cell disclosed in Japanese Translation of PCT International Application, Publication No. 2001-512290. FIG. 2 shows a sectional surface along the line A-A′ of FIG. 1. On a semiconductor substrate 11, there is provided an active region 12 configured with a boron-doped P-type semiconductor for example. On this active region 12, there are provided a drain 13 and a source 14 fabricated from a phosphorous-doped N-type semiconductor on both sides of a channel 15 for example. On the channel 15 there is formed a three-layered electric charge capturing film 16 for accumulating electric charges. This electric charge capturing film 16 comprises an insulting film 16a in contact with the channel, an insulating film 16c in contact with a gate electrode 17, and a non-conductive electric charge capturing film 16b, held between these insulating films. For reading and writing data, pin contacts 18 are respectively connected to the drain 13 and the source 14.
The non-conductive electric charge capturing film 16b forms one of bits 19a and 19b in a region in the proximity of the drain 13 or in a region in the proximity of the source 14. In FIG. 2, when a voltage is applied from the source 14 to the drain 13 to carry out writing, hot electrons (free electrons) enter the non-conductive electric charge capturing film 16b of the region of the bit 19b, and the information is recorded. On the other hand, when a voltage is applied from the drain 13 to the source 14 to carry out writing, hot electrons enter the non-conductive electric charge capturing film 16b of the region of the bit 19a, and the information (bit data) is recorded. If a voltage is not applied when writing, hot electrons are not captured. In this way, bit information of “0” and “1” are recorded.
When reading bit information, a voltage is applied in the direction opposite to writing. For example, when reading the information stored in the region of the bit 19a, a voltage is applied from the source 14 to the drain 13. When an electric charge is captured electric current does not flow, and when an electric charge is not captured, electric current flows. Therefore, the bit information can be read. By reversing the writing direction and the reading direction, a single memory cell can hold two bits of data.
However, despite the fact that a single memory cell has become able to record two bits of data and a flash memory has become common, there is still a demand for miniaturization and cost reduction in a flash memory. Accordingly, there is a strong demand for improving the level of integration.